The purpose of this study is to determine if the thickened cardiac walls of patients with hypertrophic cardiomyopathy (HC) are due to increased size or number of myocytes of increased amounts of fibrous tissue. Eight patients aged 18-42 years, who died from complications of HC and 8 age-matched controls without heart disease were studied. A 1.5 cm3 of tissue was removed from the left ventricular (LV) free wall, the right ventricular free wall and the ventricular septum (VS). Each region of each wall was evaluated for fibrous tissue by point counting; cell diameter was measured using an ocular micrometer disc. Cell layers were counted across the walls. The results revealed that increased cell size, cell layers and fibrous tissue are characteristic of HC but only in the VS are all 3 significantly increased. The fibrous tissue was most extensive in the VS (19 plus/minus 9%), but it was greater than in the controls in all 3 walls. Cell diameters were largest in the layers closest to the LV cavity.